MIMO has recently attracted much attention as a broadband wireless mobile communication technology. Especially MIMO increases spectral efficiency in proportion to the number of antennas, compared to Single-Input Single-Output (SISO) with which the spectral efficiency is difficult to achieve.
MIMO is a multiple antenna technology aiming at high-data rate communications by use of a plurality of antennas. MIMO schemes may be classified into MIMO spatial multiplexing and MIMO spatial diversity according to whether the same or different data are transmitted.
MIMO spatial multiplexing scheme is a transmission scheme in which different data are simultaneously transmitted through a plurality of transmit/receive antennas. That is, a transmitter transmits different data through different transmit antennas and a receiver distinguishes the transmitted data by appropriate interference cancellation and signal processing. Therefore, data rate is increased as much as the number of the transmit antennas.
MIMO spatial diversity scheme achieves transmit diversity by transmitting the same data through a plurality of transmit antennas. That is, spatial diversity is a kind of space-time channel coding. Spatial diversity may maximize a transmit diversity gain (a performance gain) by transmitting the same data through a plurality of transmit antennas. Yet, spatial diversity scheme is a technology for increasing transmission reliability with a diversity gain rather than a technology for increasing data rate.
The MIMO technology may also be categorized into open-loop MIMO and closed-loop MIMO depending on whether the receiver feeds back radio channel information to the transmitter. In the open-loop MIMO scheme, the transmitter transmits different data through a plurality of transmit antennas at a high data rate, if it estimates current radio channels to be available for MIMO spatial multiplexing based on whatever little available information without perfect knowledge of the statuses of the radio channels. If the transmitter estimates the current radio channels to be unavailable for MIMO spatial multiplexing, it uses the plurality of transmit antennas for diversity, thereby increasing transmission reliability.
On the other hand, the closed-loop MIMO scheme increases MIMO efficiency by allowing the receiver to transmit appropriate feedback information about radio channels to the transmitter. Specifically, the transmitter increases MIMO efficiency by performing antenna grouping, antenna selection, and precoding for a plurality of antennas based on the feedback information.
Meanwhile, a cooperative MIMO mode is defined to provide multiple antenna service to a receiver such as a Mobile Station (MS) located at a cell boundary through Base Stations (BSs) of multiple cells under a multi-cell environment. The cooperative MIMO mode replies on the idea that better performance is achieved with lower correlations among the characteristic values of channels established among a plurality of antennas. In other words, the cooperative MIMO mode utilizes the fact that as transmit antennas are spaced farther from each other and as receive antennas are spaced farther from each other, more favorable channel characteristics for spatial multiplexing are achieved.
FIG. 1 conceptually illustrates a conventional cooperative MIMO operation in a multi-cell environment. Referring to FIG. 1, a first MS (MS 1) receives a communication service from a first BS (BS 1) being its serving BS. Second and third MSs (MS 2 and MS 3) also receive communication services from their serving BSs, BS 2 and BS 3, respectively. MS 1, MS 2 and MS 3 are located at the boundaries of cells managed by their serving BSs, BS 1, BS 2 and BS 3. This means that MS 1, MS 2 and MS 3 receive interference signals from other BSs as well as signals from their serving BSs.
The cooperative MIMO mode enables such a user terminal as is located at a cell boundary and thus vulnerable to interference from neighbor cells to receive the same signal transmitted by its serving BS from the BSs of the neighbor cells, using the antennas of the neighbor cells as MIMO transmit antennas. In this manner, MIMO spatial diversity or MIMO spatial multiplexing are effected.
FIG. 2 illustrates a conventional cooperative MIMO mode in a closed-loop MIMO scheme using codebook-based precoding. As is known, a transmitter (e.g. a BS) using the precoding-based closed-loop MIMO scheme performs precoding prior to transmission of transmission data through antennas. The precoding involves multiplying the transmission data to be transmitted through the transmit antennas by a predetermined weight matrix according to a radio channel status estimated based on feedback information about the radio channel status (e.g. a Channel Quality Indicator (CQI)) received from a receiver in order to compensate for signal distortion that occurs during transmission of the data on radio channels. It is not ideal to represent all radio channel statuses with the feedback radio channel information from the receiver (i.e. radio channel information in FIG. 2) because the feedback information is too huge in amount.
In this context, both the transmitter and the receiver define a predetermined number of radio channel statuses, label them with indexes, and share the index information. This is called a codebook scheme. Then the receiver notifies the transmitter of the index of a radio channel status most approximate to a current radio channel status. The transmitter generates an appropriate weight matrix for the transmit antennas based on the received index and multiplies transmission data by the weight matrix. The precoding scheme using the indexes of a finite number of radio channel statuses is referred to as codebook-based precoding.
Referring to FIG. 2, M BSs 210-1 to 210-M (BS 1 to BS M) estimate information H1, H2, . . . , HM about radio channels established between BS 1 to BS M and k MSs 220-1 to 220-k (MS 1 to MS k) and independently generate weight matrices W1, W2, . . . , WM each having antenna weights based on the estimated radio channel information. Each of BS 1 to BS M transmits a plurality of independent data streams to a plurality of MSs or a particular MS after precoding the data streams in its precoder 230-1, . . . , or 230-M.
Hn denotes radio channel information between an nth BS and one or more MSs communicating with the nth BS. That is, Hn represents radio channels between the nth BS and one or more MSs serviced by the nth BS, on the assumption that the nth BS may be a serving BS or a cooperative BS operating in the cooperative MIMO mode for the one or more MSs. In the illustrated case of FIG. 2, since the cooperative MIMO operation is performed using codebook-based precoding, a BS detects radio channel statuses referring to indexes indicated by feedback information (radio channel information in FIG. 2) received from one or more MSs.
A scheduler 250 transmits control information indicating BSs through which data are to be transmitted and an MS to receive the data according to feedback information representing radio channel statuses and cooperative MIMO request information of MSs and BSs that are received through a backbone 200. A user data distributor 260 determines a transmission path to transmit data to a particular mobile station or a plurality of mobile stations through one or more BSs in the cooperative MIMO mode.
As described above, when the precoding-based closed-loop cooperative MIMO scheme is implemented, transmission of Preferred precoding codebook Matrix Indexes (PMIs) to BSs may degrade cooperative MIMO performance due to inter-cell interference and channel changes.